OP, i thought it was my seatbel extender too but really it was the door seals...i fixed it by getting 3m friction tape...and putting about 3 layers of it on the seal thats on the car..not the seal on the door...i didnt hear it again...for that rattle at least...1 down and 35 to go....OCD

This is one of the few rattles I had... Fixed it by treating the seals with gummiphledge. Did this twice and it never returned. My 2011.75 doesn't have this issue and it feels as though the seal material has changed from my 2010. Maybe it hasn't but it feels different.